VCSELs are widely used as light sources for optical interconnect devices, storage area networks, and sensors. In order to enable VCSELs to operate at increasingly higher speeds, or data rates, the aperture sizes must be made increasingly smaller. Decreasing the size of the aperture, however, makes the VCSEL increasingly susceptible to electrostatic discharge (ESD) damage. The ESD damage threshold for VCSELs is commonly characterized by models such as the human body model (HBM) and the machine model (MM). For a VCSEL aperture diameter in the range of 5-10 micrometers (microns), the HBM damage threshold voltage is typically in the range of 100 to 200 volts (V) and the MM damage threshold voltage is typically under 50 V. VCSELs have a p-intrinsic-n (PIN) structure and the damage threshold for ESD is asymmetric, i.e., an ESD pulse traveling in the reverse-bias direction is more damaging than an ESD pulse traveling in the forward-bias direction.
It is known to integrate a protection diode with a laser diode in a semiconductor device. For example, U.S. Pat. Nos. 6,185,240, 7,508047 and 7,693,201 disclose semiconductor devices in which a laser diode and a protection diode are integrated together in the semiconductor device. One of the problems associated with integrating the protection diode together with the laser diode in the same semiconductor device is that the inclusion of the protection diode introduces capacitance, which decreases the operating speed of the laser diode. The capacitance Cd of the protection diode can be expressed as:Cd=∈A/d,  (Equation 1)where ∈ is the permittivity of the semiconductor material, A is the area of protection diode, and d is the width of depletion region of the protection diode. Decreasing the area, A, or increasing the width, d, of the depletion region will decrease the capacitance, Cd, of the protection diode. Decreasing area A to reduce Cd is not desirable because a small area A leads to a high thermal resistance and high current density. The high thermal resistance will lead to rapid temperature rise during an ESD event and result in a low damage threshold. On the other hand, increasing the area, A, of the protection diode increases the damage threshold voltage of the laser diode, but also increases the amount of capacitance that is introduced by the protection diode, which limits the operating speed, or the data rate, of the laser diode.
The most common configuration of a VCSEL is a conducting n-type substrate with an n-type distributed Brag reflector (DBR), an active region (intrinsic layer), and a p-type DBR sequentially grown on it. Although the design described in U.S. Pat. No. 6,185,240 can be configured such that Cd is relatively small, the design cannot be used in the common VCSEL configuration described above because the cathode (n-side) of the VCSEL and diode are always connected through the substrate. Consequently, the substrate cannot be a conducting substrate.
Accordingly, a need exists for a semiconductor device having a VCSEL and a protection diode integrated together therein in a configuration that has reduced capacitance to enable the VCSEL to operate at higher speeds. A need also exists for such a configuration that is applicable to VCSELs fabricated on both conducting and semi-insulating substrates.